Preview at WWDC likely in June, followed by September launch.
iOS 6 Maps with 'Flyover' 3D Hacked to Run on iPhone 4
But as reported by SlashGear, a member of Russian site iguides has posted videos [Google translation] showing the 3D maps and Flyover working on a jailbroken iPhone 4. The poster has also included instructions for adding the functionality, although the procedure is not yet a completely straightforward one.
Apple has historically placed hardware restrictions on certain features of its iOS release, based at least in part on how well older hardware handles the specific features. In other cases like Siri on the iPhone 4S, Apple's limitations may have more to do with differentiation and marketing. But it seems reasonable to think that 3D map performance on the pre-A5 devices like the iPhone 4 simply doesn't meet Apple's requirements, and thus the company has elected not to officially support the feature on that hardware.
Regardless of the reasons for Apple's limitations on mapping features, hackers and developers will undoubtedly continue working on ways to circumvent those restrictions, and we will likely see other features like turn-by-turn navigation also come to older devices for those willing to tweak their devices to enable them.